1. Field of the Invention
The present invention relates to a semiconductor device including an oxide semiconductor and a manufacturing method thereof.
In this specification, semiconductor devices refer to all types of devices which can function by utilizing semiconductor characteristics; an electro-optical device, a semiconductor circuit, and an electronic device are all semiconductor devices.
2. Description of the Related Art
Many transistors formed over a glass substrate or the like are manufactured using amorphous silicon, polycrystalline silicon, or the like, as typically seen in liquid crystal display devices. Although transistors including amorphous silicon have low field-effect mobility, they can be formed over a larger glass substrate. On the other hand, although transistors including polycrystalline silicon have high field-effect mobility, they are not suitable for being formed over a larger glass substrate.
Other than a transistor formed including silicon, a technique in which a transistor is manufactured using an oxide semiconductor and applied to an electronic device or an optical device has attracted attention. For example, a technique in which a transistor is manufactured using zinc oxide or In—Ga—Zn-based oxide as an oxide semiconductor, and the transistor is used as a switching element of a pixel of a display device is disclosed in Patent Document 1 and Patent Document 2.
In an oxide semiconductor, part of hydrogen behaves as a donor and releases electrons as carriers. When the carrier concentration in the oxide semiconductor becomes high, a channel is formed in the transistor without voltage application to its gate. Therefore, the threshold voltage of the transistor shifts in the negative direction and it is difficult to control the threshold voltage.
In Patent Document 3, it is disclosed that by adding hydrogen into an oxide semiconductor film, the electrical conductivity of the oxide semiconductor film is increased by four to five orders of magnitude approximately. It is also disclosed that hydrogen is diffused into the oxide semiconductor film from an insulating film which is in contact with the oxide semiconductor film.